Thermal fatigue-resistant airfoil shapes from dispersion-modified metals



United States Patent 3,354,683 THERMAL FATIGUE-RESISTANT AIRFOIL SHAPES FROM DISPERSION-MODIFIED METALS David B. Arnold, Ellicott City, Md., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed Dec. 2, 1965, Ser. No. 511,213 1 Claim. (Cl. 72206) This invention relates to airfoil shapes, especially shapes which are highly resistant to thermal shock and are fabricated from rolled, thoria-strengthened, nickel sheet, and to processes for their production. More particularly the invention is directed to such processes wherein the airfoil shapes are fabricated from rolled thoria-strengthened, nickel sheet by bending the sheet, the axis of bend being substantially parallel to the direction of rolling which produced the major reduction in the sheet gauge, and to the improved airfoil shapes so produced.

Recent developments in aircraft technology have led to new demands for metals which can withstand extremely high temperatures for prolonged periods. To be most useful the metals must exhibit high strength at elevated temperatures, be resistant to creep at high temperatures and be chemically inert to atmospheric conditions, especially to oxygen-containing gases; and in addition, must be resistant to thermal fatigue and thermal shock. Nickel having particulate refractory oxides such as thoria dispersed therein is highly useful in these capacities.

In the description of this invention when reference is made to thoria-strengthened nickel it will be understood to mean nickel having a dispersed therein from 1.8 to 2.6 volume percent of thoria of particle size ranging from 10 m to 100 mu average diameter. Thermal shock is defined as the development of a steep temperature gradient and accompanying high stresses within a structure. Thermal stresses are stresses in metal resulting from nonuniform temperature distribution. Thermal fatigue is defined as fracture resulting from the presence of temperature gradients which vary with time in such a man- .ner as to produce cyclic stresses in the structure. The term formability is defined as the relative ease with which a metal can be shaped through plastic deformation.

A particular metal which has been found to possess many of the preferred properties for operation in airfoil shapes and which is especially valuable for use for prolonged periods at elevated temperatures is a nickel product known commercially as TD Nickel. This product comprises a dispersion of about two volume percent of thoria in a matrix of nickel, and is produced by E. I. du Pont de Nemours & Company. Because of its outstanding high temperature properties, thoria dispersion-modified nickel has become a valuable raw material for the production of airfoil shapes. It has now been found that airfoil shapes with enhanced resistance to thermal shock can be produced by fabrication from TD nickel rolled sheet if certain limitations are observed as hereinbelow set forth.

Normal shop practice in the fabrication of airfoil shapes from superalloy metals is to bend the metal along an axis perpendicular to the rolling direction of the superalloy sheet. In contradistinction to this, the formability of rolled TD Ni sheet is excellent in both directions as demonstrated by the ability of the material to withstand without cracking bending through an angle of 90 around a diameter equal to two times the nominal thickness of the material. Nevertheless, it has now been found that airfoil shapes formed from TD Ni sheet exhibit anisotropy as evidenced by thermal fatigue, dependent upon the direction of axis of the bend effected in the fabrication of the airfoil shape.

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Now according to the present invention it has been found that the thermal shock resistance of airfoil vanes fabricated from rolled, thoria-strengthened nickel sheet is greatly improved providing the axis of the bend made to form the shape is in the direction parallel to the direction of rolling which has produced the major reduction in the sheet gauge. By major reduction is meant that the total of the reductions in gauge effected by rolling in one direction is greater than percent of the total reduction effected in forming of the final rolled sheet from the ingot.

To illustrate the process of this invention, and the benefits derived therefrom, airfoil shapes were formed according to the process just described, using TD nickel sheet of 30, 40, 50, and mils thickness. In each case the sheet was produced by rolling in one direction only; no cross-rolling was effected at any point in the reduction of the sheet.

Airfoil shapes were prepared from each of these sheets by bending the sheet according to the invention so that the axis of bend was parallel to the major rolling direction; for comparison airfoil shapes were made with bends perpendicular to the axis of rolling. The high temperature properties of these airfoil vanes were determined by cyclic testing, by alternate heating and cooling of the vanes over periods ranging from one-half to two minutes, depending on the gauge of the sheet used to form the vane. One test cycle was considered to be the total of the time required to heat the airfoil shape to the indicated temperature, as measured by an optical pyrometer reading, plus an equal period of cooling by a blast of cold air directed against the airfoil shapes. Failure of any vane was determined by visual examination, the point of failure being the appearance of cracks visible without magnification.

The following table shows the very substantial increase in thermal shock resistance obtained in the vanes fabricated according to the processes of this invention (parallel bends), when compared with vanes fabricated from identical material and tested in an identical manner but fabricated according to prior art methods (perpendicular bends). In these tests the bends made in the airfoil shapes were approximately 2.5Tthat is, the diameter of the bend was 2.5 times the norminal thickness of the sheet.

The following table shows the results of testing of the airfoil shapes formed according to this invention as compared with known prior art methods.

Gauge Axis of Bend Relative Test Temp. Cycles of Thermal of Sheet to Major Rolling F.) Test Shock Test Direction Results 0.030 Perpendicular 2,1 TFF. P allel 350 TFF. 300 TFF. 500 N o TI F 300 N o TFF 50 N0 TFF o 050 {Perpendicular 2,100 400 TFF.

Parallel 500 No TFF Plus 2,200 300 N0 'IFF Plus 2,300. 50 No TFF 0 060 {Perpendicular 2,100 440 TFF.

' Parallel 2,100 500 N0 TFF Plus 2,200 300 No 'IFF Plus 2,300 50 No TFF 0 075 {Perpendieular 2,100 247 FF.

' Parallel 2,100 500 No TFF Plus 2,200. 200 No TFF Plus 2,300. 50 No TFF 1 TFF=Thermal Fatigue Failure.

From the above results it will be seen that excellent high temperature properties are exhibited by the airfoil I claim:

In a process for producing an airfoil shape for high temperature use the steps comprising rolling a sheet of thoria-strengthened nickel to the desired gauge and thereafter bending the rolled sheet along an axis substantially parallel to the axis of the rolling direction Which has efiected the maximum reduction in the gauge of said sheet.

4 References Cited UNITED STATES PATENTS 3,290,915 12/1966 Cornell 72-240 5 RICHARD J. HERBST, Primary Examiner.

H. D. HOINKES, Assistant Examiner. 

